Method and Apparatus for Growth of High Quality Carbon Single-Walled Nanotubes

ABSTRACT

Method and processes for synthesizing single-wall carbon nanotubes is provided. A carbon precursor gas is contacted with metal catalysts deposited on a support material. The metal catalysts are preferably nanoparticles having diameters less than about 50 nm. The reaction temperature is selected such that it is near the eutectic point of the mixture of metal catalyst particles and carbon.

RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. Provisional Application 60/763,813, filed on Jan. 30, 2006, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to methods for the preparation (synthesis) of carbon single-walled nanotubes using chemical vapor deposition method.

BACKGROUND

Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Tijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. They reported carbon nanotubes having up to seven walls. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al Nature 363:603 (1993); Bethune et al, Nature 363: 605 23085-12560 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.

Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.

Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.

Single-walled carbon nanotubes have been produced by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus (Saito et al. Chem. Phys. Lett. 236: 419 (1995)). Further, the use of mixtures of transition metals has been shown to increase the yield of single-walled carbon nanotubes in the arc discharge apparatus. However, the yield of nanotubes is still low, the nanotubes can exhibit significant variations in structure and size (properties) between individual tubes in the mixture, and the nanotubes can be difficult to separate from the other reaction products. In a typical arc discharge process, a carbon anode loaded with catalyst material (typically a combination of metals such as nickel/cobalt, nickel/cobalt/iron, or nickel and transition element such as yttrium) is consumed in arc plasma. The catalyst and the carbon are vaporized and the single-walled carbon nanotubes are grown by the condensation of carbon onto the condensed liquid catalyst. Sulfur compounds such as iron sulfide, sulfur or hydrogen sulfides are typically used as catalyst promoter to maximize the yield of the product.

A typical laser ablation process for producing single-walled carbon nanotubes is disclosed by Andreas Thess et al. (1996). Metal catalyst particle such as nickel-cobalt alloy is mixed with graphite powder at a predetermined percentage, and the mixture is pressed to obtain a pellet. A laser beam is radiated to the pellet. The laser beam evaporates the carbon and the nickel-cobalt alloy, and the carbon vapor is condensed in the presence of the metal catalyst. Single-wall carbon nanotubes with different diameters are found in the condensation. However, the addition of a second laser to their process which give a pulse 50 nanoseconds after the pulse of the first laser favored the (10,10) chirality (a chain of 10 hexagons around the circumference of the nanotube). The product consisted of fibers approximately 10 to 20 nm in diameter and many micrometers long comprising randomly oriented single-wall nanotubes, each nanotube having a diameter of about 1.38 nm.

Many researchers consider chemical vapor deposition as the only viable approach to large scale production and for controllable synthesis of carbon single walled nanotubes (Dai et al. (Chem. Phys. Lett 260: 471 (1996), Hafner et al., Chem. Phys. Lett. 296: 195 (1998), Su. M., et al. Chem. Phys. Lett., 322: 321 (2000)). Typically, the growth of carbon SWNTs by CVD method is conducting at the temperatures 550-1200° C. by decomposition of hydrocarbon gases (methane, ethylene, alcohol, . . . ) on metal nanoparticles (Fe, Ni, Co, . . . ) supported by oxide powders. The diameters of the single-walled carbon nanotubes vary from 0.7 nm to 3 nm. The synthesized single-walled carbon nanotubes are roughly aligned in bundles and woven together similarly to those obtained from laser vaporization or electric arc method. The use of metal catalysts comprising iron and at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Mo and W), VII (Mn, Tc and Re) or the lanthanides has also been proposed (U.S. Pat. No. 5,707,916).

Presently, there are two types of chemical vapor deposition for the syntheses of single-walled carbon nanotubes that are distinguishable depending on the form of supplied catalyst. In one, the catalyst is embedded in porous material or supported on a substrate, placed at a fixed position of a furnace, and heated in a flow of hydrocarbon precursor gas. Cassell et al. (1999) J. Phys. Chem. B 103: 6484-6492 studied the effect of different catalysts and supports on the synthesis of bulk quantities of single-walled carbon nanotubes using methane as the carbon source in chemical vapor deposition. They systematically studied Fe(NO₃)₃ supported on Al₂O₃, Fe(SO₄)₃ supported on Al₂O₃, Fe/Ru supported on Al₂O₃, Fe/Mo supported on Al₂O₃, and Fe/Mo supported on Al₂O₃—SiO₂ hybrid support. The bimetallic catalyst supported on the hybrid support material provided the highest yield of the nanotubes. Su et al. (2000) Chem. Phys. Lett. 322: 321-326 reported the use of a bimetal catalyst supported on an aluminum oxide aerogel to produce single-walled carbon nanotubes. They reported preparation of the nanotubes is greater than 200% the weight of the catalyst used. In comparison, similar catalyst supported on Al₂O₃ powder yields approximately 40% the weight of the starting catalyst. Thus, the use of the aerogel support improved the amount of nanotubes produced per unit weight of the catalyst by a factor of 5.

In the second type of carbon vapor deposition, the catalyst and the hydrocarbon precursor gas are fed into a furnace using the gas phase, followed by the catalytic reaction in a gas phase. The catalyst is usually in the form of a metalorganic. Nikolaev et al. (1999) Chem. Phys. Lett. 313: 91 disclose a high-pressure CO reaction (HiPCO) method in which carbon monoxide (CO) gas reacts with the metalorganic iron pentacarbonyl (Fe(CO)₅) to form single-walled carbon nanotubes. It is claimed that 400 g of nanotubes can be synthesized per day. Chen et al. (1998) Appl. Phys. Lett. 72: 3282 employ benzene and the metalorganic ferrocene (Fe(C₅H₅)₂) delivered using a hydrogen gas to synthesize single-walled carbon nanotubes. The disadvantage of this approach is that it is difficult to control particles sizes of the metal catalyst. The decomposition of the organometallic provides disordered carbon (not desired) the metal catalyst having variable particle size that results in nanotubes having a wide distribution of diameters and low yields.

In another method, the catalyst is introduced as a liquid pulse into the reactor. Ci et al. (2000) Carbon 38: 1933-1937 dissolve ferrocene in 100 mL of benzene along with a small amount of thiophene. The solution is injected into a vertical reactor in a hydrogen atmosphere. The technique requires that the temperature of bottom wall of the reactor had to be kept at between 205-230° C. to obtain straight carbon nanotubes. In the method of Ago et al. (2001) J. Phys. Chem. 105: 10453-10456, colloidal solution of cobalt:molybdenum (1:1) nanoparticles is prepared and injected into a vertically arranged furnace, along with 1% thiophene and toluene as the carbon source. Bundles of single-walled carbon nanotubes are synthesized. One of the disadvantages of this approach is the very low yield of the nanotubes produced.

It is generally recognized that smaller catalyst particles of less than 3 nm are preferred for the growth of smaller diameter carbon nanotubes. However, the smaller catalyst particles easily aggregate at the higher temperatures required for the synthesis of carbon nanotubes. U.S. Patent Application No. 2004/0005269 to Huang et al. discloses a mixture of catalysts containing at least one element from Fe, Co, and Ni, and at least one supporting element from the lanthanides. The lanthanides are said to decrease the melting point of the catalyst by forming alloys so that the carbon nanostructures can be grown at lower temperatures.

Aside from the size of the catalyst, the temperature of the reaction chamber can also be important for the growth of carbon nanotubes. U.S. Pat. No. 6,764,874 to Zhang et al. discloses a method of preparing nanotubes by melting aluminum to form an alumina support and melting a thin nickel film to form nickel nanoparticles on the alumina support. The catalyst is then used in a reaction chamber at less than 850° C. U.S. Pat. No. 6,401,526, and U.S. Patent Application Publication No. 2002/00178846, both to Dai et al., disclose a method of forming nanotubes for atomic force microscopy. A portion of the support structure is coated with a liquid phase precursor material that contains a metal-containing salt and a long-chain molecular compound dissolved in a solvent. The carbon nanotubes are made at a temperature of 850° C.

Thus, it is well known that the diameter of the SWNTs produced is proportional to the size of the catalyst particle. In order to synthesize nanotubes of small diameter, it s necessary to have catalyst particles of very small particle size (less than about 1 nm). Catalysts of small particle size are difficult to synthesize, and even with small catalyst particle sizes, a distribution of catalyst sizes is obtained which results in the formation of nanotubes with a range of diameters.

One solution to the synthesis of uniform diameter nanotubes is to use a template, such as molecular sieves, that have a pore structure which is used to control the distribution of catalyst size and thereby the size of the SWNTs formed. Thus, the diameter of SWNT can be changed by changing the pore size of the template. These methods are not versatile. Thus, there is a need for methods and processes for controllable synthesis of carbon single walled nanotubes with small and narrow distributed diameters. Accordingly, the present invention provides novel methods and processes for the synthesis of SWNTs with small and narrow distributed diameters.

SUMMARY

The present invention provides methods and processes for growing single-wall carbon nanotubes. In one aspect, a carbon precursor gas and metal catalysts on supports are heated to a reaction temperature near the eutectic point (liquid phase) of the metal-carbon phase. Further, the reaction temperature is below the melting point of the metal catalysts.

In one aspect, the methods involve contacting a carbon precursor gas with a catalyst on a support at a temperature near the eutectic point of the catalyst-carbon phase wherein SWNT are formed. The carbon precursor gas can be methane that can additionally contain other gases such as argon and hydrogen. The catalyst can be a V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, or a transition metal or combinations thereof. The catalyst preferably has a particle size between about 1 nm to about 50 nm. The catalyst can be supported on a powdered oxide, such as Al₂O₃, SiO₃, MgO and the like, herein the catalyst and the support are in a ratio of about 1:1 to about 1:50. The SWNTs are produced by employing a reaction temperature that is about 5° C. to about 150° C. above the eutectic point.

In another aspect, the invention provides a carbon nanotube structure produced by the process of contacting a carbon precursor gas with a catalyst on a support at a temperature between the melting point of the catalyst and the eutectic point of the catalyst and carbon. The carbon precursor gas can be methane that can additionally contain other gases such as argon and hydrogen. The catalyst can be a V metal, a Group VI metal, a Group VII metal, a Group VIII metal, a lanthanide, or a transition metal or combinations thereof. The catalyst preferably has a particle size between about 1 nm to about 15 nm. The catalyst can be supported on a powdered oxide, such as Al₂O₃, SiO₃, MgO and the like, wherein the catalyst and the support are in a ratio of about 1:1 to about 1:50.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. A) Evolution of hydrogen concentration during carbon SWNTs growth on Fe:Al₂O₃ (1:15 molar ratio) catalyst. Insets: sequential introduction of C¹² and C¹³ isotopes, for 3 min and 17 min (a1); 7 min and 13 min (a2) and 13 min and 7 min (a3), respectively. B) Evolution of melting point of Fe catalyst (solid circles) and Fe:Mo (1:0.21 molar ratio, open squares) during carbon SWNTs growth measured by DSC technique. C) Evolutions of Raman I_(G)/I_(D) ratios for carbon SWNTs growth on Fe catalyst and carbon uptake dependence on synthesis duration. Inset: Evolution of I_(G)/I_(D) ratios for the carbon SWNTs growth on Fe:Mo (1:0.21 molar ratio) catalyst.

FIG. 2. Raman radial breathing and tangential modes for carbon SWNTs synthesized on Fe and Fe:Mo catalysts by using sequential introduction of C¹² and C¹³ isotopes.

FIG. 3. A) Hydrogen concentration evolution at 820° C. for Al₂O₃; Fe:Al₂O₃ (1:15 molar ratio); Mo:Al₂O₃ (0.21:15, molar ratio) and Fe:Mo:Al₂O₃ (1:0.21:15 molar ratio) samples.

FIG. 4. Hydrogen concentration evolution dependence on reactor temperature for Al₂O₃ support material, for Mo:Al₂O₃; Fe:Al₂O₃ and Fe:Mo:Al₂O₃ catalysts, respectively.

DETAILED DESCRIPTION

Unless otherwise stated, the following terms used in this application, including the specification and claims, have the definitions given below. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Definition of standard chemistry terms may be found in reference works, including Carey and Sundberg (1992) “Advanced Organic Chemistry 3^(rd) Ed.” Vols. A and B, Plenum Press, New York, and Cotton et al. (1999) “Advanced Inorganic Chemistry 6^(th) Ed.” Wiley, New York.

The terms “single-walled carbon nanotube” or “one-dimensional carbon nanotube” are used interchangeable and refer to cylindrically shaped thin sheet of carbon atoms having a wall consisting essentially of a single layer of carbon atoms, and arranged in a hexagonal crystalline structure with a graphitic type of bonding.

The term “multi-walled carbon nanotube” as used herein refers to a nanotube composed of more than one concentric tubes.

The terms “metalorganic” or “organometallic” are used interchangeably and refer to co-ordination compounds of organic compounds and a metal, a transition metal or metal halide.

The term “eutectic point” refers to the lowest possible temperature of solidification for an alloy, and can be lower than that of any other alloy composed of the same constituents in different proportions.

The catalyst composition may be any catalyst composition known to those of skill in the art that is routinely used in chemical vapor deposition processes. The function of the catalyst in the carbon nanotube growth process is to decompose the carbon precursors and aid the deposition of ordered carbon. The method, processes, and apparatuses of the present invention preferably use metal nanoparticles as the metallic catalyst. The metal or combination of metals selected as the catalyst can be processed to obtain the desired particle size and diameter distribution. The metal nanoparticles can then be separated by being supported on a material suitable for use as a support during synthesis of carbon nanotubes using the metal growth catalysts described below. As known in the art, the support can be used to separate the catalyst particles from each other thereby providing the catalyst materials with greater surface area in the catalyst composition. Such support materials include powders of crystalline silicon, polysilicon, silicon nitride, tungsten, magnesium, aluminum and their oxides, preferably aluminum oxide, silicon oxide, magnesium oxide, or titanium dioxide, or combination thereof, optionally modified by addition elements, are used as support powders. Silica, alumina and other materials known in the art may be used as the support, preferably alumina is used as the support.

The metal catalyst can be selected from a Group V metal, such as V or Nb, and mixtures thereof, a Group VI metal including Cr, W, or Mo, and mixtures thereof, VII metal, such as, Mn, or Re, Group VIII metal including Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and mixtures thereof, or the lanthanides, such as Ce, Eu, Er, or Yb and mixtures thereof, or transition metals such as Cu, Ag, Au, Zn, Cd, Sc, Y, or La and mixtures thereof. Specific examples of mixture of catalysts, such as bimetallic catalysts, which may be employed by the present invention include Co-Cr, Co-W, Co-Mo, Ni-Cr, Ni-W, Ni-Mo, Ru—Cr, Ru-W, Ru-Mo, Rh—Cr, Rh-W, Rh-Mo, Pd-Cr, Pd-W, Pd-Mo, Ir-Cr, Pt-Cr, Pt-W, and Pt-Mo. Preferably, the metal catalyst is iron, cobalt, nickel, molybdenum, or a mixture thereof, such as Fe-Mo, Co-Mo and Ni-Fe-Mo.

The metal, bimetal, or combination of metals can be used to prepare metal nanoparticles having defined particle size and diameter distribution. The metal nanoparticles can be prepared using the literature procedure described in described in Harutyunyan et al., NanoLetters 2, 525 (2002). Alternatively, the catalyst nanoparticles can be prepared by thermal decomposition of the corresponding metal salt added to a passivating salt, and the temperature of the solvent adjusted to provide the metal nanoparticles, as described in the co-pending and co-owned U.S. patent application Ser. No. 10/304,316, or by any other method known in the art. The particle size and diameter of the metal nanoparticles can be controlled by using the appropriate concentration of metal in the passivating solvent and by controlling the length of time the reaction is allowed to proceed at the thermal decomposition temperature. Metal nanoparticles having particle size of about 0.01 nm to about 20 nm, more preferably about 0.1 nm to about 3 nm and most preferably about 0.3 nm to 2 nm can be prepared. The metal nanoparticles can thus have a particle size of 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 nm, and up to about 20 nm. In another aspect, the metal nanoparticles can have a range of particle sizes. For example, the metal nanoparticles can have particle sizes in the range of about 3 nm and about 7 nm in size, about 5 nm and about 10 nm in size, or about 8 nm and about 16 nm in size. The metal nanoparticles can optionally have a diameter distribution of about 0.5 nm to about 20 nm, preferably about 1 nm to about 15 nm, more preferably about 1 nm to about 5 nm. Thus, the metal nanoparticles can have a diameter distribution of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 nm.

The metal salt can be any salt of the metal, and can be selected such that the melting point of the metal salt is lower than the boiling point of the passivating solvent. Thus, the metal salt contains the metal ion and a counter ion, where the counter ion can be nitrate, nitride, perchlorate, sulfate, sulfide, acetate, halide, oxide, such as methoxide or ethoxide, acetylacetonate, and the like. For example, the metal salt can be iron acetate (FeAc₂), nickel acetate (NiAc₂), palladium acetate (PdAc₂), molybdenum acetate (MoAc₃), and the like, and combinations thereof. The melting point of the metal salt is preferably about 5° C. to 50° C. lower than the boiling point, more preferably about 5° C. to about 20° C. lower than the boiling point of the passivating solvent.

The metal salt can be dissolved in a passivating solvent to give a solution, a suspension, or a dispersion. The solvent is preferably an organic solvent, and can be one in which the chosen metal salt is relatively soluble and stable, and where the solvent has a high enough vapor pressure that it can be easily evaporated under experimental conditions. The solvent can be an ether, such as a glycol ether, 2-(2-butoxyethoxy)ethanol, H(OCH₂CH₂)₂—O—(CH₂)₃CH₃, which will be referred to below using the common name diethylene glycol mono-n-butyl ether, and the like.

The relative amounts of metal salt and passivating solvent are factors in controlling the size of nanoparticles produced. A wide range of molar ratios, here referring to total moles of metal salt per mole of passivating solvent, can be used for forming the metal nanoparticles. Typical molar ratios of metal salt to passivating solvent include ratios as low as about 0.0222 (1:45), or as high as about 2.0 (2:1), or any ratio in between. Thus, for example, about 5.75×10⁻⁵ to about 1.73×10⁻³ moles (10-300 mg) of FeAc₂ can be dissolved in about 3×10⁻⁴ to about 3×10⁻³ moles (50-500 ml) of diethylene glycol mono-n-butyl ether.

In another aspect, more than one metal salt can be added to the reaction vessel in order to form metal nanoparticles composed of two or more metals, where the counter ion can be the same or can be different. The relative amounts of each metal salt used can be a factor in controlling the composition of the resulting metal nanoparticles. For the bimetals, the molar ratio of the first metal salt to the second metal salt can be about 1:10 to about 10:1, preferably about 2:1 to about 1:2, or more preferably about 1.5:1 to about 1:1.5, or any ratio in between. Thus, for example, the molar ratio of iron acetate to nickel acetate can be 1:2, 1:1.5, 1.5:1, or 1:1. Those skilled in the art will recognize that other combinations of metal salts and other molar ratios of a first metal salt relative to a second metal salt may be used in order to synthesize metal nanoparticles with various compositions.

The passivating solvent and the metal salt reaction solution can be mixed to give a homogeneous solution, suspension, or dispersion. The reaction solution can be mixed using standard laboratory stirrers, mixtures, sonicators, and the like, optionally with heating. The homogenous mixture thus obtained can be subjected to thermal decomposition in order to form the metal nanoparticles.

The thermal decomposition reaction is started by heating the contents of the reaction vessel to a temperature above the melting point of at least one metal salt in the reaction vessel. Any suitable heat source may be used including standard laboratory heaters, such as a heating mantle, a hot plate, or a Bunsen burner, and the heating can be under reflux. The length of the thermal decomposition can be selected such that the desired size of the metal nanoparticles can be obtained. Typical reaction times can be from about 10 minutes to about 120 minutes, or any integer in between. The thermal decomposition reaction is stopped at the desired time by reducing the temperature of the contents of the reaction vessel to a temperature below the melting point of the metal salt.

The size and distribution of metal nanoparticles produced can be verified by any suitable method. One method of verification is transmission electron microscopy (TEM). A suitable model is the Phillips CM300 FEG TEM that is commercially available from FEI Company of Hillsboro, OR. In order to take TEM micrographs of the metal nanoparticles, 1 or more drops of the metal nanoparticle/passivating solvent solution are placed on a carbon membrane grid or other grid suitable for obtaining TEM micrographs. The TEM apparatus is then used to obtain micrographs of the nanoparticles that can be used to determine the distribution of nanoparticle sizes created.

The metal nanoparticles, such as those formed by thermal decomposition described in detail above, can then be supported on solid supports. The solid support can be silica, alumina, MCM-41, MgO, ZrO₂, aluminum-stabilized magnesium oxide, zeolites, or other oxidic supports known in the art, and combinations thereof. For example, Al₂O₃—SiO₂ hybrid support could be used. Preferably, the support is aluminum oxide (Al₂O₃) or silica (SiO₂). The oxide used as solid support can be powdered thereby providing small particle sizes and large surface areas. The powdered oxide can preferably have a particle size between about 0.01 μm to about 100 μm, more preferably about 0.1 μm to about 10 μm, even more preferably about 0.5 μm to about 5 μm, and most preferably about 1 μm to about 2 μm. The powdered oxide can have a surface area of about 50 to about 1000 m²/g, more preferably a surface area of about 200 to about 800 m²/g. The powdered oxide can be freshly prepared or commercially available.

In one aspect, the metal nanoparticles are supported on solid supports via secondary dispersion and extraction. Secondary dispersion begins by introducing particles of a powdered oxide, such as aluminum oxide (Al₂O₃) or silica (SiO₂), into the reaction vessel after the thermal decomposition reaction. A suitable Al₂O₃ powder with 1-2 μm particle size and having a surface area of 300-500 m²/g is commercially available from Alfa Aesar of Ward Hill, MA, or Degussa, NJ. Powdered oxide can be added to achieve a desired weight ratio between the powdered oxide and the initial amount of metal used to form the metal nanoparticles. Typically, the weight ratio can be between about 10:1 and about 15:1. For example, if 100 mg of iron acetate is used as the starting material, then about 320 to 480 mg of powdered oxide can be introduced into the solution.

The mixture of powdered oxide and the metal nanoparticle/passivating solvent mixture can be mixed to form a homogenous solution, suspension or dispersion. The homogenous solution, suspension or dispersion can be formed using sonicator, a standard laboratory stirrer, a mechanical mixer, or any other suitable method, optionally with heating. For example, the mixture of metal nanoparticles, powdered oxide, and passivating solvent can be first sonicated at roughly 80° C. for 2 hours, and then sonicated and mixed with a laboratory stirrer at 80° C. for 30 minutes to provide a homogenous solution.

After secondary dispersion, the dispersed metal nanoparticles and powdered oxide can be extracted from the passivating solvent. The extraction can be by filtration, centrifugation, removal of the solvents under reduced pressure, removal of the solvents under atmospheric pressure, and the like. For example, extraction includes heating the homogenized mixture to a temperature where the passivating solvent has a significant vapor pressure. This temperature can be maintained until the passivating solvent evaporates, leaving behind the metal nanoparticles deposited in the pores of the Al₂O₃. For example, if diethylene glycol mono-n-butyl ether as the passivating solvent, the homogenous dispersion can be heated to 231° C., the boiling point of the passivating solvent, under an N₂ flow. The temperature and N₂ flow are maintained until the passivating solvent is completely evaporated. After evaporating the passivating solvent, the powdered oxide and metal nanoparticles are left behind on the walls of the reaction vessel as a film or residue. When the powdered oxide is Al₂O₃, the film will typically be black. The metal nanoparticle and powdered oxide film can be removed from the reaction vessel and ground to create a fine powder, thereby increasing the available surface area of the mixture. The mixture can be ground with a mortar and pestle, by a commercially available mechanical grinder, or by any other methods of increasing the surface area of the mixture will be apparent to those of skill in the art.

Without being bound by any particular theory, it is believed that the powdered oxide serves two functions during the extraction process. The powdered oxides are porous and have high surface area. Therefore, the metal nanoparticles will settle in the pores of the powdered oxide during secondary dispersion. Settling in the pores of the powdered oxide physically separates the metal nanoparticles from each other, thereby preventing agglomeration of the metal nanoparticles during extraction. This effect is complemented by the amount of powdered oxide used. As noted above, the weight ratio of metal nanoparticles to powdered oxide can be between about 1:10 and 1:15, such as, for example, 1:11, 1:12, 2:25, 3:37, 1:13, 1:14, and the like. The relatively larger amount of powdered oxide in effect serves to further separate or ‘dilute’ the metal nanoparticles as the passivating solvent is removed. The process thus provides metal nanoparticles of defined particle size.

As will be apparent to those of skill in the art, the catalyst thus prepared can be stored for later use. In another aspect, the metal nanoparticles can be previously prepared, isolated from the passivating solvent, and purified, and then added to a powdered oxide in a suitable volume of the same or different passivating solvent. The metal nanoparticles and powdered oxide can be homogenously dispersed, extracted from the passivating solvent, and processed to increase the effective surface area as described above. Other methods for preparing the metal nanoparticle and powdered oxide mixture will be apparent to those skilled in the art.

The metal nanoparticles thus formed can be used as a growth catalyst for synthesis of carbon nanotubes, nanofibers, and other one-dimensional carbon nanostructures by a chemical vapor deposition (CVD) process.

The carbon nanotubes can be synthesized using carbon precursors, such as carbon containing gases. In general, any carbon containing gas that does not pyrolize at temperatures up to 800° C. to 1000° C. can be used. Examples of suitable carbon-containing gases include carbon monoxide, aliphatic hydrocarbons, both saturated and unsaturated, such as methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene and propylene; oxygenated hydrocarbons such as acetone, and methanol; aromatic hydrocarbons such as benzene, toluene, and naphthalene; and mixtures of the above, for example carbon monoxide and methane. In general, the use of acetylene promotes formation of multi-walled carbon nanotubes, while CO and methane are preferred feed gases for formation of single-walled carbon nanotubes. The carbon-containing gas may optionally be mixed with a diluent gas such as hydrogen, helium, argon, neon, krypton and xenon or a mixture thereof.

The methods and processes of the invention provide for the synthesis of SWNTs with a narrow distribution of diameters. The narrow distribution of carbon nanotube diameters is obtained by activating small diameter catalyst particles preferentially during synthesis by selecting the lowest eutectic point as the reaction temperature.

In one aspect of the invention, the metal nanoparticles supported on powdered oxides can be contacted with the carbon source at the reaction temperatures according to the literature methods described in Harutyunyan et al., NanoLetters 2, 525 (2002). Alternatively, the metal nanoparticles supported on the oxide powder can be aerosolized and introduced into the reactor maintained at the reaction temperature. Simultaneously, the carbon precursor gas is introduced into the reactor. The flow of reactants within the reactor can be controlled such that the deposition of the carbon products on the walls of the reactor is reduced. The carbon nanotubes thus produced can be collected and separated.

The metal nanoparticles supported on the oxide powder can be aerosolized by any of the art known methods. In one method, the supported metal nanoparticles are aerosolized using an inert gas, such as helium, neon, argon, krypton, xenon, or radon. Preferably argon is used. Typically, argon, or any other gas, is forced through a particle injector, and into the reactor. The particle injector can be any vessel that is capable of containing the supported metal nanoparticles and that has a means of agitating the supported metal nanoparticles. Thus, the catalyst deposited on a powdered porous oxide substrate can be placed in a beaker that has a mechanical stirrer attached to it. The supported metal nanoparticles can be stirred or mixed in order to assist the entrainment of the catalyst in the transporter gas, such as argon.

The growth technique, enhanced by an attached mass spectrometer for in-situ parametrical studies, enables us to elucidate the evolution of catalyst activity during carbon single walled nanotubes (SWNTs) growth and in this manner reveal the catalyst features and their relationship with the growth conditions. Any changes of catalyst features due to the composition modification, diameter variation or interaction with support material we were detected by monitoring catalyst activity. By variation of synthesis temperature, duration and carbon feedstock, type of transport gas and pressure we exposed their relationship with catalyst activity and in this manner with catalyst features and thereby reveal the optimum condition for growth of high quality carbon SWNTs.

The elucidation of catalyst particle specific features favorable for carbon single-walled nanotubes (SWNTs) growth will help to control over the characteristics of grown nanotubes and eventually will promote exploitation of their unique properties (R. H. Baughman, A. A. Zakhidov, W. A. De Heer, Science 297, 787 (2002)). There are a large number of studies regarding the influence of catalyst composition on SWNTs growth, where most often 3d metals and their combinations have been considered. It is already established that catalyst preparation methods, pretreatments, diameters, crystallographic and electronic structures, and abilities of carbide and oxide formations have influence on nanotube growth. These studies are extended by reports about the essential role of catalyst-supports coupling (more common supports Al, Zr, Mg, Si based oxides). In addition, it was found that the synthesis parameters have an impact on the thermodynamics and kinetics of the growth. Nevertheless, intense research to reveal the common features of the catalysts and corresponding synthesis parameters favorable for nanotube growth from described complexity is still underway.

To evaluate catalyst activity we measured the evolution of hydrogen concentration during carbon SWNTs growth, appeared as a result of catalytic decomposition of hydrocarbon, by using a mass-spectrometer (Thermo Star GSD 300T, with SEM Detector) attached to the outlet of the gas stream of the CVD apparatus. Then current corresponding to the mass m=2 was measured which is prortional to the molecules concentration. The catalytic decomposition of methane results the proportional numbers of hydrogen molecules and carbon atoms. Of course there are the possibilities of formation also other hydrocarbons CxHy. However, the analogical measurements by using in parallel also Gas Chromatography (GC-17A, SHIMADZU), with monitoring the presents of the other molecules C₂H₂, C₂H₆, C₂H₄, CO and CO₂ in the stream have shown that the main product of the decomposition was H molecules. Therefore, the evolution of carbon atoms concentration is analogical to the evolution behavior of formed hydrogen molecules.

Before introduce into the reactor the gases were passed through a purification cartridge of specific purifier (Praxair) in order to trap residual O₂ and H₂O. After catalyst reduction the reactor was thoroughly purged about 3 hours by Ar gas in order to remove residual H₂ and He from the reactor. For calibration and comparison of independent measurements the exit stream of the reactor was fitted by N₂ as a standard gas. Along with the hydrogen also CH₄, H₂O, gases were monitored.

Thus, any changes of catalyst features which influence on catalyst electronic structure and in this manner on hydrocarbon decomposition efficiency and eventually on the kinetics of nanotube growth were detected by monitoring of hydrogen concentration. Analogously, influence of synthesis parameters was also revealed.

The carbon SWNTs were grown by passing a mixture of methane (60 cm³/min, Praxair, 99,999%) diluted in argon (200 cm³/min) over the Fe catalyst particles (with molar ratio Fe:Al₂O₃=1:1 5) at 820° C. for 90 min as described in the literature (A. R. Harutyunyan, B. K. Pradhan, U. J. Kim, G. Chen, P. C. Eklund, Nano Letters, 2, 525 (2002)). The growth of nanotubes was independently confirmed by transmission electron microscopy and Raman measurements. The rapid increase of H₂ concentration until t˜7±11 min (FIG. 1A) was followed by a slowly return to the almost constant value, which consistent with concentration of non catalytic decomposition of CH₄ (FIG. 2A). On the other hand, the DSC studies of the independent samples, synthesized under analogical experimental conditions but different synthesis durations (3; 5; 7; 20 and 90 min), revealed solid-liquid (when t≦7+2 min) and liquid-solid (when t≧20 min) phase transitions of the catalyst induced by carbon atoms diffusion into the catalyst, and formation of Fe-C phases, respectively (FIG. 1B) (H. Kanzow, A. Ding, Phys. Rev. B 60, 11180 (1999); A. R Harutyunyan, E. Mora, T. Tokune Applied Phys. Lett. 87, 051919 (2005)). Comparison of these results (FIGS. 1A and B) shows that the increases of catalyst activity coincidences with liquefaction process of catalyst, while the liquid-solid phase transition initiates deactivation of catalyst. Interestingly, the Raman spectroscopy (λ=785 nm) studies show dramatic increases of the ratio between the intensities of grown SWNT's G-band and D-band (I_(G)/I_(D)), which is a measure of the graphitic order in the carbon deposit, in the same t<7 to 10 min interval, where the catalyst is in liquid phase and possess high activity (FIG. 1C), and about 70 wt % of overall carbon yield (wt % carbon relative to the Fe/alumina catalyst) also was gained in the same interval of time. To help establish the relationship between observed evolution of catalyst features and nanotube growth, along with C¹²H₄ gas, methane gas with C¹³ isotope (C¹³H₄, 99.99%, Cambridge Isotope Lab. Inc.) was sequentially introduced in the intervals of time when catalyst is liquefied and possess high activity and as well as when catalyst begins to solidify and looses the activity. A series of samples were prepared by using the methane gas C¹²H₄ for the first 3 min, 7 min (catalyst still liquefied) and 13 min (catalyst solidified) with following introductions of the C¹³H₄ gas for 17 min, 13 min and 7 min respectively (insets in FIG. 1A: A1, A2, A3). The Raman spectra of carbon SWNTs obtained by using methane gas with C¹³ isotope is identical to the spectra with C¹² isotope. The only principal difference is that the Raman shift frequency is √12/13 times smaller because the heavier carbon atoms result smaller phonon energies. The Raman spectra for the sample synthesized using the C¹²H₄ for the first 3 min with following introduction C¹³H₄ for 17 min, contains significant contribution corresponding to the SWNTs with C¹³ atoms, while for the sample feed with C¹²H₄ for first 7 min of growth duration and then 13 min with C¹³H₄, this contribution decreases (FIG. 2B, C). Finally the spectrum for the sample with 13 min duration of C¹²H₄ source and 7 min C¹³H₄ is completely identical with the spectrum of nanotubes with only C¹² isotope. Comparison of these results with catalyst activity and DSC measurements, (FIGS. 1A,B,C) allow to conclude, that the liquefied catalyst is favorable for carbon SWNTs growth and the catalyst lifetime, favorable for growth for this particular synthesis conditions, is τ≈7-10 min. Moreover, one of the reasons for growth termination is the solidification of catalyst through the formation of stable carbide phases.

It is well known that the addition of Mo to Fe catalyst makes it more efficient for SWNTs production. The combined studies described above were repreated using Fe:Mo:Al₂O₃ (with common molar ratio 1:0.21:15) catalyst under analogically synthesis conditions. The first distinctive feature observed is that detected activity of Fe: Mo: Al₂O₃ catalyst was dramatically higher of that for Fe:Al₂O₃ (FIG. 3) for during all synthesis duration, and the formation of carbon SWNTs occurs at earlier stages of synthesis. According to previous studies, the nanotube growth does not begin immediately after the introduction of hydrocarbon gas, but require certain carbon concentration to be dissolved in and diffuse through the particle before the growth can start. The temperature and concentration gradients are the main driving forces for this process. In CVD reactions the temperature gradient appears because of exothermal hydrocarbon decomposition reaction, which is not the case in our experiment because we use the CH₄ gas. So, the threshold concentration gradient of carbon atoms has been reached rapidly in case of Fe/Mo catalyst due to the high catalyst activity and probably less activation energy of diffusion. Second, the evolution behavior of Fe:Mo:Al₂O₃ (1:0.21:15) catalyst is qualitatively and quantitatively different compare with mathematical sum of Fe:Al₂O₃ (1:15) and Mo:Al₂O₃ (molar ratio 0.21:15) catalysts. This fact was attributed to the intermetallic interaction between Mo and Fe with possible formation of Fe-Mo alloy. Moreover, DSC measurements show that the catalyst is still in liquid state even for nanotube growth duration up to 90 min (FIG. 1B) and therefore is able to produce nanotubes. Indeed, in contrast to Fe catalyst the Raman spectra for the nanotubes obtained by using Fe/Mo catalyst and sequential introduction of C¹²H₄ for 13 min with following introduction of C¹³H₄ for 7 min show clear contribution of C¹³ atoms (FIG. 2). However, the catalyst does not show activity when t>30 min (FIG. 3A) and no any contributions from C¹³ atoms were found in the Raman spectra of nanotubes when C¹³H₄ was introduced into the reactor following C¹²H₄ after t≧20 min. So, even though the catalyst was found liquefied till 90 min it does not results nanotube growth when t≧20 min. This fact attributed to the formation of various form of disordered Sp carbon along with nanotube growth, which covers the surface of catalyst and eventually deactivates it. Thus, the addition of Mo importantly prevents the solidification because of carbide formation, and as a result prolongs the catalyst lifetime favorable for nanotube growth almost 2 times.

Even though the catalyst features are favorable for nanotube growth still it is important to know appropriate synthesis conditions. The activity of catalyst on synthesis parameters was evaluated. The evolution of hydrogen concentration dependence on reactor temperature for the Al₂O₃; Fe:Al₂O₃; and Fe:Mo:Al₂O₃ samples for fixed other parameters is shown in FIG. 4. As one can see that the thermal decomposition temperature of used particular carbon source (CH₄) limits the synthesis temperature from the supreme. It is obvious that in case of Tsynthesis>830° C., the contribution of carbon atoms formed because of thermal decomposition will be significant and may rapidly poison the catalyst and affect on quality of tubes by coating the catalyst surface and tubes walls, respectively. On the other hand adding the Mo results the higher decomposition rate of carbon formation (increases the catalyst activity) for given temperature compare with pure Fe catalyst.

Without being bound to theory, mass spectrometer attached into the CVD technique may offer opportunities for parametrical studies of catalyst features during carbon SWNTs growth, by in-situ evolution of catalyst activity. By monitoring hydrogen concentration occurred because of hydrocarbon decomposition it is possible to reveal the a) catalyst lifetime for nanotube growth; b) to find the appropriate support material for given catalyst composition and diameter; c) establish the optimal catalyst composition and size for growth of carbon nanotube; d) establish the optimal synthesis temperature which leads growth of high quality carbon nanotube by excluding of formation of amorphous carbon; e) establish the carbon feedstock rate for growth of high quality carbon SWNTs; f) establish the appropriate pressure of gases inside the reactor favorable for carbon SWNTs growth; g) establish the composition of transport gases, including the oxidizers and reducers.

While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. All printed patents and publications referred to in this application are hereby incorporated herein in their entirety by this reference. 

1. A chemical vapor deposition method for the preparation of single-wall carbon: nanotubes (SWNT), the method comprising: contacting a carbon precursor gas with a catalyst on a support at a temperature less than the melting point of the catalyst and about 5° C. to about 150° C. above the eutectic point of the catalyst wherein SWNT are formed.
 2. The method of claim 1, wherein the carbon precursor gas is methane.
 3. The method of claim 2, wherein the carbon precursor gas further comprises an inert gas and hydrogen.
 4. The method of claim 3, wherein the inert gas is argon, helium, nitrogen, hydrogen, or combinations thereof.
 5. The method of claim 1, wherein the catalyst is iron, molybdenum, or combinations thereof.
 6. The method of claim 1, wherein the catalyst has a particle size between 1 nm to 10 nm.
 7. The method of claim 6, wherein the catalyst has a particle size of about 1 nm.
 8. The method of claim 6, wherein the catalyst has a particle size of about 3 nm.
 9. The method of claim 6, wherein the catalyst has a particle size of about 5 nm.
 10. The method of claim 6, wherein the support is a powdered oxide selected from the group consisting of Al₂O₃, SiO₃, MgO and zeolites.
 11. The method of claim 10, wherein the powdered oxide is Al₂O₃.
 12. The method of claim 1, wherein the temperature is about 10° C. to about 100° C. above the eutectic point.
 13. The method of claim 1, wherein the temperature is about 50° C. above the eutectic point.
 14. The method of claim 13, wherein the temperature is about 80° C. above the eutectic point.
 15. The method of claim 1, wherein the SWNTs have a diameter of about 0.8 nm to about 2 nm.
 16. A single-wall carbon nanotube (SWNT) produced by the process of: contacting a carbon precursor gas with a catalyst on a support selected from the group consisting of Al₂O₃, SiO₃, MgO and zeolite; and maintaining reaction temperature between the melting point of the catalyst and the eutectic point of the catalyst.
 17. The process of claim 16, wherein the carbon precursor gas is methane.
 18. The process of claim 17, wherein the carbon precursor gas further comprises an inert gas and hydrogen.
 19. The process of claim 18, wherein the inert gas is argon, helium, nitrogen, hydrogen, or combinations thereof.
 20. The process of claim 16, wherein the catalyst is iron, molybdenum, or combinations thereof
 21. The process of claim 16, wherein the catalyst has a particle size between 1 nm to 10 nm.
 22. The process of claim 16, wherein the powdered oxide is Al₂O₃.
 23. The process of claim 16, wherein the catalyst and the support are in a ratio of about 1:1 to about 1:50.
 24. The process of claim 16, wherein the temperature is about 10° C. to about 100° C. above the eutectic point.
 25. The process of claim 16, wherein the temperature is about 50° C. above the eutectic point.
 26. The process of claim 16, wherein the temperature is about 80° C. above the eutectic point. 